Early mouse embryos cultured in vitro will be exposed to DNA- and RNA-tumor viruses and the effects of viral infection will be followed in short-term experiments in vitro and in long-term experiments after transfer of infected embryos into the uteri of foster mothers. For example, studies already in progress on SV-40 viral infection will continue with regard to the following problems: electron microscopy of the intracellular events in viral infection; integration of the SV-40 genome into mouse nuclear DNA as detected by nucleic acid hybridization; and expression of the viral genome in mouse embryo cells. The endogenous A- and C-type particles of early mouse embryos will be further characterized by the following techniques and approaches: immuno-electron microscopy using antiserum to A particle capsid protein; localization of presumptive viral RNA species by subcellular fractionation; and hybridization of these RNAs with complementary DNA to tumor viral RNAS.